1. Field of the Invention
This invention relates generally to photographic cameras and, more particularly, to cameras that include a film drive system for moving a photographic filmstrip within the camera.
2. Description of the Related Art
In conventional-wind cameras, a light-tight film canister containing a photographic filmstrip is placed in a recess of the camera for loading. The end of the filmstrip extending from the canister is pulled across an exposure gate and coupled to a take-up spool. After loading, the filmstrip is intermittently extracted from the canister and is wound onto the take-up spool as the photographer uses frames of the filmstrip by completing exposures. The filmstrip extracted from the canister is protected from ambient light by a camera back door and associated encasements.
If the camera back door is opened in mid-roll, the used frames on at least the outer-most wraps of film on the take-up spool will be ruined. Opening the back door permits ambient light to strike the filmstrip and fog at least the outermost wraps of the filmstrip on the take-up spool. The actual number of frames ruined will vary, depending on the amount of ambient light, the amount of time the camera back was open, and the light sensitivity of the filmstrip.
To prevent ruining used filmstrip frames due to accidental openings of camera backs, cameras have been provided with a pre-wind feature. Pre-wind cameras include a motorized film transport mechanism that winds substantially all of the filmstrip onto the take-up spool before any frames are exposed. After the filmstrip is wound onto the take-up spool, the filmstrip is intermittently wound back into the light-tight film canister by one frame length as each frame is exposed. Thus, in a pre-wind camera, the exposed frames are protected within the film canister if the camera back is opened in mid-roll, with the possible exception of the exposed frame typically located between the camera exposure gate and the film canister.
Some pre-wind cameras respond to opening of the camera back in mid-roll by automatically setting the camera frame counter to zero and continuously rewinding the filmstrip back into the film canister. U.S. Pat. No. 4,676,621 to Molloy-Desormeaux, for example, describes a pre-wind camera that responds to the opening of the camera back in mid-roll by automatically initializing the frame counter and continuously rewinding the remaining unused filmstrip back into the film canister until a sensor indicates that film is no longer on the take-up spool. This system is said to ensure that the film transport mechanism will not be prematurely halted before the used frames of the filmstrip are completely wound back into the canister.
The pre-wind camera described in U.S. Pat. No. 4,687,311 to Molloy-Desormeaux does not change the frame counter if the camera back is opened in mid-roll but does rewind the filmstrip back into the canister. Unfortunately, the camera does not assist the user in accurately assessing the number of frames likely to have been fogged and therefore the user would not be inclined to try to use any remaining unfogged/unexposed film. The camera user would have to re-load the filmstrip onto the take-up spool to begin picture taking, rewind the appropriate number of frames back into the film canister and trip the shutter button with the back closed and the objective lens covered to move the film to the point at which the camera back was opened, and then rewind the filmstrip by an appropriate number of additional frames until the fogged frames are bypassed.
Occasionally, a camera user will accidentally open the camera back during the pre-winding operation when the filmstrip is being wound onto the take-up spool. Unfortunately, pre-wind cameras typically are not directed to respond to an instance of opening a camera back during the pre-wind operation. It can be very difficult for a camera user to estimate the frame number when the camera back was opened, estimate the number of frames fogged, and advance the filmstrip to the appropriate location to bypass the fogged frames.
From the discussion above, it should be apparent that there is a need for a camera that can detect the opening of the camera back, determine the appropriate number of filmstrip frames likely to have been fogged, and automatically skip the appropriate number of frames, thus saving a good portion of a filmstrip that otherwise might have been discarded, without requiring intervention on the part of the photographer. The present invention satisfies this need.